A large number of designs for cable anchors have been disclosed in the prior art to form a "dead-end" coupling for securing a cable against a load carried on an oil drilling derrick or the like. The cable is commonly formed from wire and is trained or roved about a crown block and traveling block in the derrick to raise and lower the drill pipe and casing by means of the cable.
Massive loads tend to be present in deep drilling operations and it is accordingly important for the cable anchor to be able to carry them.
It is also common practice to provide a supply drum for additional cable which is slipped or fed through the cable anchor for replacing the cable roving in the derrick, usually after the cable experiences substantial wear.
A cable anchor assembly of this type was disclosed in U.S. Pat. No. 2,282,685 issued to Timbs on May 12, 1942. The cable anchor assembly of that patent included a snubbing drum and associated clamping means arranged between the derrick and the supply drum for operation in the manner generally referred to above.
U.S. Pat. No. 2,488,070 issued November 15, 1949 to Spalding further disclosed the combination of a weight indicator or load cell employed in combination with a cable anchor assembly to indicate the amount of load carried on the cable.
Still further refinements in cable anchor assemblies have been provided in the prior art. For example, U.S. Pat. No. 3,973,435 issued August 10, 1976 to Decker disclosed a cable anchor assembly including the formation of fluting or spiral grooving in a cylindrical surface of the snubbing drum with the cable being arranged within the groove.
Such cable anchor assemblies have been found to be satisfactory particularly where bare wire cable is employed with metal-to-metal engagement existing between the cable and snubbing drum and between the cable and clamp.
However, more recently, there has been a tendency to employ plastic coated cable or rope in applications other than oil well drilling for a number of reasons set forth, for example, in U.S. Pat. No. 3,824,777 issued July 23, 1974 to Riggs. That patent disclosed a stranded wire rope including a heavy viscous lubricant in its core with the outer portions of the rope or cable being impregnated or coated with plastic. This cable is commonly referred to as plastic filled valley (PFV) rope or cable and has been found to be desirable for use in drilling applications for a number of reasons. Initially, use of the internally lubricated rope or cable avoids the need for externally applying grease to the cable. Furthermore, plastic coated cables such as the PFV rope referred to above increases wear life or ton miles in drilling operations and the like while also protecting the wire from the environment, including corrosion effects, etc.
However, the use of plastic coated rope in drilling operations has been found to be incompatible with cable anchor assemblies designed for use with bare wire cable. lnitially, decreased frictional engagement with the drum and with the clamp exists because of the plastic-to-wire engagement and also because of the tendency for the plastic coated rope to maintain its cylindrical shape in cross section when wound about the drum and carrying a heavy load.
A number of design variations were considered in development of the present invention in an attempt to overcome this problem and to assure that the load is secured by the cable anchor assembly while still permitting the rope to be slipped through the cable anchor assembly as desired or necessary.
For example, in some instances, it was found necessary to remove a portion of the plastic coating in order to obtain normal metal-to-metal engagement between the rope and drum for developing sufficient friction.
It has also been found that additional turns of rope could be formed about the drum for the same purpose. However, the additional turns of rope unduly increased the axial length of the drum making the cable assembly more bulky and tending to interfere with the drilling operation. At the same time, the presence of additional turns about the drum also interfered with the slipping or feeding of rope from the supply drum.
Still another expedient for overcoming this problem was the provision of increased clamping surface associated with the drum for securing the plastic coated cable. However, this was also found to be generally undesirable because of the tendency for clamps to deform the cable and prevent its proper operation as roving associated with a derrick.
Accordingly, there was found to remain a need for substantial improvements in the cable anchor assembly in order to firmly secure the rope and carry the full load, without damage to the rope, while also permitting the rope to be selectively slipped or fed through the cable anchor assembly when it was necessary for example to replace rope in the derrick.